Could you help me understand what a problem domain is and how can I build my class diagram based on it?

So I just started studying Software Engineering because I am really interested in it and my professor in London asked us to create an app which is like Instagram (only theoretically, without the actual implementation of code) and I would like some help on something.

I have started with writing and studying different parts, like risk analysis and everything and now I just created a class diagram with the classes that are in my opinion essential for the project. Like content, photos, users, display, etc. What I really haven’t understood is that we needed to draw the class diagram based on the problem domain of my case.

Maybe it’s because of my lower English level, but problem sounds literal to me and while some websites mention that the problem domain relates to the risks and the problems of your project, others say that the problem domain is whatever is essential for the creation of the app.

Can someone please elaborate on that? You will really help me get deeper into the project because it’s very confusing and they haven’t explained it so much at uni yet..

Thanks 🙂